The World Wide Web uses relatively simple technologies with sufficient scalability, efficiency and utility that they have resulted in a remarkable information space of interrelated resources, growing across languages, cultures, and media. In an effort to preserve these properties of the information space as the technologies evolve, this architecture document discusses the core design components of the Web. They are identification of resources, representation of resource state,
and the protocols that support the interaction between agents and resources in the space. We relate core design components, constraints, and good practices to the principles and properties they support.

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This is NOT the 16 August 2004 Last Call Working Draft of "Architecture of the World Wide Web, First Edition." The Last Call review period ends 17 September 2004, at 23:59 EDT. Please send Last Call review comments on this document before that date to the public W3C TAG mailing list public-webarch-comments@w3.org (archive). Last
Call Working Draft status is described in section 7.4.2 of the W3C Process Document. To the extent possible, please provide a separate email message for each distinct comment.

The TAG has used the last call comments page to track the status and discussion of comments on the 9 December 2003 Draft. This draft has been informed by a large number of comments made on the on the 9 December 2003 Last Call Working Draft. Because the text has changed substantially, it is not always clear whether or not comments
made on the previous draft still apply. The TAG expects a future revision of this document to become a W3C Recommendation.

The TAG charter describes a process for issue resolution by the TAG. In accordance with those provisions, the TAG maintains a running issues list. The First Edition of "Architecture of the World Wide Web" does not address every issue that the TAG has accepted since it began work in January 2002. The TAG has selected a subset of issues that the First
Edition does address to the satisfaction of the TAG; those issues are identified in the TAG's issues list. The TAG intends to address the remaining (and future) issues after publication of the First Edition as a Recommendation.

This document uses the concepts and terms regarding URIs as defined in draft-fielding-uri-rfc2396bis-07, preferring them to those defined in RFC 2396. The IETF Internet Draft draft-fielding-uri-rfc2396bis-07 will has been approved as a full standard, obsoleting RFC 2396, which is the current URI standard. The TAG is tracking the evolution of draft-fielding-uri-rfc2396bis-07.

Publication as a does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

The World Wide Web (WWW, or simply Web) is an information space in which the items of interest, referred to as resources, are identified by global identifiers called Uniform Resource Identifiers (URI).

Examples such as the following travel scenario are used throughout this document to illustrate typical behavior of Web agents—people or software acting on this information space. A user agent acts on behalf of a user. Software agents include servers, proxies, spiders, browsers, and multimedia players.

Story

While planning a trip to Mexico, Nadia reads “Oaxaca weather information: 'http://weather.example.com/oaxaca'” in a glossy travel magazine. Nadia has enough experience with the Web to recognize that "http://weather.example.com/oaxaca" is a URI and that she is likely to be able to retrieve associated information with her web browser. When Nadia enters the URI into her browser:

The browser recognizes that what Nadia typed is a URI.

The browser performs an information retrieval action in accordance with its configured behavior for resources identified via the "http" URI scheme.

The authority responsible for "weather.example.com" provides information in a response to the retrieval request.

The browser interprets the response, identified as XHTML by the server, and performs additional retrieval actions for inline graphics and other content as necessary.

The browser displays the retrieved information, which includes hypertext links to other information. Nadia can follow these hypertext links to retrieve additional information.

This scenario illustrates the three architectural bases of the Web that are discussed in this document:

Identification (§2). URIs are used to identify resources. In this travel scenario, the resource is a periodically updated report on the weather in Oaxaca, and the URI is “http://weather.example.com/oaxaca”.

Interaction (§3). Web agents communicate using standardized protocols that enable interaction through the exchange of messages that adhere to a defined syntax and semantics. By entering a URI into a retrieval dialog or selecting a hypertext link, Nadia tells her browser to perform a retrieval action for the resource identified by the URI. In this example, the browser sends an HTTP GET request to the server at "weather.example.com", via
TCP/IP port 80, and the server sends back a message containing what it determines to be a representation of the resource as of the time that representation was generated. Note that this example is specific to hypertext browsing of information—other kinds of interaction are possible, both within browsers and through the use of other types of Web agent; our example is intended to illustrate one common interaction, not define the range of possible interactions or limit the ways in which agents might use the
Web.

Formats (§4). Most protocols used for representation retrieval and/or submission make use of a sequence of one or more messages, which taken together contain a payload of representation data and metadata, to transfer the representation between agents. The choice of interaction protocol places limits on the formats of representation data and metadata that can be transmitted. HTTP, for example, typically transmits a single octet stream plus
metadata, and uses the "Content-Type" and "Content-Encoding" header fields to further identify the format of the representation. In this scenario, the representation transferred is in XHTML, as identified by the "Content-type" HTTP header field containing the registered Internet media type name, "application/xhtml+xml". That Internet media type name indicates that the representation data can be processed according to the XHTML specification.

Nadia's browser is configured and programmed to interpret the receipt of an "application/xhtml+xml" typed representation as an instruction to render the content of that representation according to the XHTML rendering model, including any subsidiary interactions (such as requests for external stylesheets or in-line images) called for by the representation. In the scenario, the XHTML representation data received from the initial request instructs Nadia's browser to also retrieve
and render in-line the weather maps, each identified by a URI and thus causing an additional retrieval action, resulting in additional representations that are processed by the browser according to their own data formats (e.g., "application/svg+xml" indicates the SVG data format), and this process continues until all of the data formats have been rendered. The result of all of this processing, once the browser has reached an application steady-state that completes Nadia's initial requested action, is
commonly referred to as a "Web page".

The following illustration shows the relationship between identifier, resource, and representation.

In the remainder of this document, we highlight important architectural points regarding Web identifiers, protocols, and formats. We also discuss some important general architectural principles (§5) in the context of the Web.

This document describes the properties we desire of the Web and the design choices that have been made to achieve them. It promotes re-use of existing standards when suitable, and gives guidance on how to innovate in a manner consistent with Web architecture.

The terms MUST, MUST NOT, SHOULD, SHOULD NOT, and MAY are used in the principles, constraints, and good practice notes in accordance with RFC 2119 [RFC2119].

This document does not include conformance provisions for these reasons:

Conforming software is expected to be so diverse that it would not be useful to be able to refer to the class of conforming software agents.

Some of the good practice notes concern people; specifications generally define conformance for software, not people.

We do not believe that the addition of a conformance section is likely to increase the utility of the document.

This document is intended to inform discussions about issues of Web architecture. The intended audience for this document includes:

Participants in W3C Activities

Other groups and individuals designing technologies to be integrated into the Web

Implementers of W3C specifications

Web content authors and publishers

Note: This document does not distinguish in any formal way the terms "language" and "format." Context determines which term is used. The phrase "specification designer" encompasses language, format, and protocol designers.

This document presents the general architecture of the Web. Other groups inside and outside W3C also address specialized aspects of Web architecture, including accessibility, quality assurance, internationalization, device independence, and Web Services. The section on Architectural Specifications (§7.1) includes references to these related specifications.

This document strives for a balance between brevity and precision while including illustrative examples. TAG findings are informational documents that complement the current document by providing more detail about selected topics. This document includes some excerpts from the findings. Since the findings evolve independently, this document also includes references to approved TAG findings. For other TAG issues
covered by this document but without an approved finding, references are to entries in the TAG issues list.

Many of the examples in this document that involve human activity suppose the familiar Web interaction model where a person follows a link via a user agent, the user agent retrieves and presents data, the user follows another link, etc. This document does not discuss in any detail other interaction models such as voice browsing (see, for example, [VOICEXML2]). For instance, when a graphical user agent running on a laptop computer or
hand-held device encounters an error, the user agent can report errors directly to the user through visual and audio cues, and present the user with options for resolving the errors. On the other hand, when someone is browsing the Web through voice input and audio-only output, stopping the dialog to wait for user input may reduce usability since it is so easy to "lose one's place" when browsing with only audio-output. This document does not discuss how the principles, constraints, and good practices
identified here apply in all interaction contexts.

An architectural principle is a fundamental rule that applies to a large number of situations and variables. Architectural principles include "separation of concerns", "generic interface", "self-descriptive syntax," "visible semantics," "network effect" (Metcalfe's Law), and Amdahl's Law: "The speed of a system is limited by its slowest component."

In the design of the Web, some choices, like the names of the p and li elements in HTML, the choice of the colon (:) character in URIs, or grouping bits into eight-bit units (octets), are somewhat arbitrary; if paragraph had been chosen instead of p or asterisk (*) instead of colon, the large-scale result would, most likely, have been the same. This document focuses on more fundamental design choices: design choices that lead to constraints, i.e.,
restrictions in behavior or interaction within the system. Constraints may be imposed for technical, policy, or other reasons to achieve desirable properties in the system, such as accessibility, global scope, relative ease of evolution, efficiency, and dynamic extensibility.

In order to communicate internally, a community agrees (to a reasonable extent) on a set of terms and their meanings. Since its inception, one goal of the Web has been to build a global community in which any party can share information with any other party. To achieve this goal, the Web makes use of a single global identification system: the URI. URIs are a cornerstone of Web architecture, providing identification that is common across the Web. The global scope of URIs
promotes large-scale "network effects": the value of an identifier increases the more it is used consistently (for example, the more it is used in hypertext links (§4.4)).

The choice of syntax for global identifiers is somewhat arbitrary; it is their global scope that is important. The Uniform Resource Identifier, [URI], has been successfully deployed since the creation of the Web. There are substantial benefits to participating in the existing network of URIs, including linking, bookmarking, caching, and indexing by search engines, and there are substantial costs to
creating a new identification system that has the same properties as URIs.

To benefit from and increase the value of the World Wide Web, agents should provide URIs as identifiers for resources.

A resource should have an associated URI if another party might reasonably want to create a hypertext link to it, make or refute assertions about it, retrieve or cache a representation of it, include all or part of it by reference into another representation, annotate it, or perform other operations on it. Software developers should expect that sharing URIs across applications will be useful, even if that utility is not initially evident. The TAG finding "URIs, Addressability, and the use of HTTP GET and POST" discusses additional benefits and considerations of URI addressability.

Note: Some URI schemes (such as the "ftp" URI scheme specification) use the term "designate" where this document uses "identify."

By design a URI identifies one resource. We do not limit the scope of what might be a resource. The term "resource" is used in a general sense for whatever might be identified by a URI. It is conventional on the hypertext web to describe web pages, images, product catalogs, etc. as “resources”. The distinguishing characteristic of these resources is that all of their essential characteristics can be conveyed in a message.
We identify this set as “information resources”.

This document is an example of an information resource. It consists of words and punctuation symbols and graphics and other artifacts that can be encoded, with varying degrees of fidelity, into a sequence of bits. There is nothing about the essential information content of this document that cannot in principle be transfered in a representation.

However, our use of the term resource is intentionally more broad. Other things, such as cars and dogs (and, if you’ve printed this document on physical sheets of paper, the artifact that you are holding in your hand), are resources too. They are not information resources, however, because their essence is not information. Although it is possible to describe a great many things about a car or a dog in a sequence of bits, the sum of those things will invariably be an
approximation of the essential character of the resource.

We define the term “information resource” because we observe that it is useful in discussions of web technology and may be useful in constructing specifications for facilities built for use on the web.

Since the scope of a URI is global, the resource identified by a URI does not depend on the context in which the URI appears (see also the section about indirect identification (§2.2.3)).

[URI] is an agreement about how the Internet community allocates names and associates them with the resources they identify. URIs are divided into schemes (§2.4) that define, via their scheme specification, the mechanism by which scheme-specific identifiers are associated with resources. For example, the "http" URI scheme ([RFC2616]) uses DNS and TCP-based HTTP servers
for the purpose of identifier allocation and resolution. As a result, identifiers such as "http://example.com/somepath#someFrag" often take on meaning through the community experience of performing an HTTP GET request on the identifier and, if given a successful response, interpreting the response as a representation of the identified resource. (See also Fragment Identifiers (§2.6).) Of course, a retrieval action like GET is not the only way to obtain information about a
resource. One might also publish a document that purports to define the meaning of a particular URI. These other sources of information may suggest meanings for such identifiers, but it's a local policy decision whether those suggestions should be heeded.

Just as one might wish to refer to a person by different names (by full name, first name only, sports nickname, romantic nickname, and so forth), Web architecture allows the association of more than one URI with a resource. URIs that identify the same resource are called URI aliases. The section on URI aliases (§2.3.1) discusses some of the potential costs of creating multiple
URIs for the same resource.

Many sections of this document address questions about the relationship between URIs and resources, including:

As discussed above, a URI identifies one resource. Using the same URI to directly identify different resources produces a URI collision. Collision often imposes a cost in communication due to the effort required to resolve ambiguities.

Suppose, for example, that one organization makes use of a URI to refer to the movie The Sting, and another organization uses the same URI to refer to a discussion forum about The Sting. To a third party, aware of both organizations, this collision creates confusion about what the URI identifies, undermining the value of the URI. If one wanted to talk about the creation date of the resource identified by the URI, for instance, it would not be clear
whether this meant "when the movie created" or "when the discussion forum about the movie was created."

The section on URI allocation (§2.2.2) examines approaches for establishing the authoritative source of information about what resource a URI identifies.

URI allocation is the process of associating a URI with a resource. Allocation can be performed both by resource owners and by third parties. It is important to avoid URI collision (§2.2.1). Social and technical solutions have been devised to help avoid assigning the same URI to different resources. However, the success or failure of these different approaches depends on the extent to which there is consensus in the Internet community
on abiding by the defining specifications.

URI ownership is a relation between a URI and a social entity, such as a person, organization, or specification. In the case of a specification, ownership ultimately lies with the community that maintains the specification. URI ownership gives the relevant social entity rights to:

pass on ownership of some or all owned URIs to another owner—URI allocation; and

to associate a resource with an owned URI—URI assignment.

By social convention, URI ownership is delegated from the IANA URI scheme registry [IANASchemes], itself a social entity, to IANA-registered URI scheme specifications. Some URI scheme specifications further delegate ownership to subordinate registries or to other nominated owners, who may further delegate ownership.

The approach taken for the "http" URI scheme, for example, follows the pattern whereby the Internet community delegates authority, via the IANA URI scheme registry and the DNS, over a set of URIs with a common prefix to one particular owner. One consequence of this approach is the Web's heavy reliance on the central DNS registry.

A different approach is taken by the URN Syntax scheme [RFC2141] which delegates ownership of portions of URN space to URN Namespace specifications which themselves are registered in an IANA-maintained registry of URN Namespace Identifiers.

URI owners are responsible for avoiding the assignment of equivalent URIs to multiple resources. Thus, if a URI scheme specification does provide for the delegation of individual or organized sets of URIs, it should take pains to ensure that ownership ultimately resides in the hands of a single social entity. Allowing multiple owners increases the likelihood of URI collisions.

URI owners may organize or deploy infrastruture to ensure that representations of associated resources are available and, where appropriate, interaction with the resource is possible through the exchange of representations.

There are social expectations for responsible representation management (§3.5) by URI owners, discussed below. Additional social implications of URI ownership are not discussed here.

See TAG issue siteData-36, which concerns the expropriation of naming authority.

Some schemes use techniques other than delegated ownership to avoid collision. For example, the specification for the data URL (sic) scheme [RFC2397] specifies that the resource identified by a data scheme URI has only one possible representation. The representation data makes up the URI that identifies that resource. Thus, the specification itself determines how data URIs are allocated; no delegation is possible.

Other schemes (such as "news:comp.text.xml") rely on a social process.

To say that the URI "mailto:nadia@example.com" identifies both an Internet mailbox and Nadia, the person, introduces a URI collision. However, we can use the URI to indirectly identify Nadia. Identifiers are commonly used in this way.

Listening to a news broadcast, one might hear a report on Britain that begins, "Today, 10 Downing Street announced a series of new economic measures." Generally, "10 Downing Street" identifies the official residence of Britain's Prime Minister. In this context, the news reporter is using it (as English rhetoric allows) to indirectly identify the British government. Similarly, URIs identify resources, but they can also be used in many constructs to indirectly identify other
resources. Globally adopted assignment policies make some URIs appealing as general-purpose identifiers. Local policy establishes what they indirectly identify.

Suppose that nadia@example.com is Nadia's email address. The organizers of a conference Nadia attends might use "mailto:nadia@example.com" to refer indirectly to her (e.g., using the URI as a database key in their database of conference participants). This does not introduce a URI collision.

URIs that are identical, character-by-character, refer to the same resource. Since Web Architecture allows the association of multiple URIs with a given resource, two URIs that are not character-by-character identical may still refer to the same resource. Different URIs do not necessarily refer to different resources but there is generally a higher computational cost to determine that different URIs refer to the same resource.

To reduce the risk of a false negative (i.e., an incorrect conclusion that two URIs do not refer to the same resource) or a false positive (i.e., an incorrect conclusion that two URIs do refer to the same resource), some specifications describe equivalence tests in addition to character-by-character comparison. Agents that reach conclusions based on comparisons that are not licensed by the relevant specifications take responsibility for any problems that result; see the
section on error handling (§5.3) for more information about responsible behavior when reaching unlicensed conclusions. Section 6 of [URI] provides more information about comparing URIs and reducing the risk of false negatives and positives.

Although there are benefits (such as naming flexibility) to URI aliases, there are also costs. URI aliases are harmful when they divide the web of related resources. A corollary of Metcalfe's Principle (the "Network Effect") is that the value of a given resource can be measured by the number and value of other resources in its network neighborhood, that is, the resources that link to it.

The problem with aliases is that if half of the neighborhood points to one URI for a given resource, and the other half points to a second, different URI for that same resource, the neighborhood is divided. Not only is the aliased resource undervalued because of this split, the entire neighborhood of resources loses value because of the missing second-order relationships that should have existed among the referring resources by virtue of their references to the aliased
resource.

A URI owner SHOULD NOT associate arbitrarily different URIs with the same resource.

URI consumers also have a role in ensuring URI consistency. For instance, when transcribing a URI, agents should not gratuitously percent-encode characters. The term "character" refers to URI characters as defined in section 2 of [URI]; percent-encoding is discussed in section 2.1 of that specification.

An agent that receives a URI SHOULD refer to the associated resource using the same URI, character-by-character.

When a URI alias does become common currency, the URI owner should use protocol techniques such as server-side redirects to relate the two resources. The community benefits when the URI owner supports redirection of an aliased URI to the corresponding "official" URI. For more information on redirection, see section 10.3, Redirection, in [RFC2616]. See also [CHIPS]
for a discussion of some best practices for server administrators.

URI aliasing only occurs when more than one URI is used to identify the same resource. The fact that different resources sometimes have the same representation does not make the URIs for those resources aliases.

Story

Dirk would like to add a link from his Web site to the Oaxaca weather site. He uses the URI http://weather.example.com/oaxaca and labels his link “weather in Oaxaca on 1 August 2004”. Nadia points out to Dirk that he is setting misleading expectations for the URI he has used. The Oaxaca weather site policy is that the URI in question identifies the current weather in Oaxaca—on any given day—and not the weather on 1 August. Of course, on the first of August in 2004,
Dirk's link will be correct, but the rest of the time he will be misleading visitors to his Web site. Nadia points out to Dirk that the weather site does make available a different URI permanently assigned to a resource describing the weather on 1 August 2004.

In this story, there are two resources: “the current weather in Oaxaca” and “the weather in Oaxaca on 1 August 2004”. The Oaxaca weather site assigns two URIs to these two different resources. On 1 August 2004, the representations for these resources are identical. That fact that deferencing two different URIs produces identical representations does not imply that the two URIs are aliases.

In the URI "http://weather.example.com/", the "http" that appears before the colon (":") names a URI scheme. Each URI scheme has a specification that explains the scheme-specific details of how scheme identifiers are allocated and become associated with a resource. The URI syntax is thus a federated and extensible naming system wherein each scheme's specification may further restrict the syntax and semantics of identifiers within that scheme.

Examples of URIs from various schemes include:

mailto:joe@example.org

ftp://example.org/aDirectory/aFile

news:comp.infosystems.www

tel:+1-816-555-1212

ldap://ldap.example.org/c=GB?objectClass?one

urn:oasis:names:tc:entity:xmlns:xml:catalog

While Web architecture allows the definition of new schemes, introducing a new scheme is costly. Many aspects of URI processing are scheme-dependent, and a large amount of deployed software already processes URIs of well-known schemes. Introducing a new URI scheme requires the development and deployment not only of client software to handle the scheme, but also of ancillary agents such as gateways, proxies, and caches. See [RFC2718] for
other considerations and costs related to URI scheme design.

Because of these costs, if a URI scheme exists that meets the needs of an application, designers should use it rather than invent one.

A specification SHOULD reuse an existing URI scheme (rather than create a new one) when it provides the desired properties of identifiers and their relation to resources.

Consider our travel scenario: should the agent providing information about the weather in Oaxaca register a new URI scheme "weather" for the identification of resources related to the weather? They might then publish URIs such as "weather://travel.example.com/oaxaca". When a software agent dereferences such a URI, if what really happens is that HTTP GET is invoked to retrieve a representation of the resource, then an "http" URI would have
sufficed.

The Internet Assigned Numbers Authority (IANA) maintains a registry [IANASchemes] of mappings between URI scheme names and scheme specifications. For instance, the IANA registry indicates that the "http" scheme is defined in [RFC2616]. The process for registering a new URI scheme is defined in [RFC2717].

Unregistered URI schemes SHOULD NOT be used for a number of reasons:

There is no generally accepted way to locate the scheme specification.

Someone else may be using the scheme for other purposes.

One should not expect that general-purpose software will do anything useful with URIs of this scheme beyond URI comparison.

One misguided motivation for registering a new URI scheme is to allow a software agent to launch a particular application when retrieving a representation. The same thing can be accomplished at lower expense by dispatching instead on the type of the representation, thereby allowing use of existing transfer protocols and implementations.

Even if an agent cannot process representation data in an unknown format, it can at least retrieve it. The data may contain enough information to allow a user or user agent to make some use of it. When an agent does not handle a new URI scheme, it cannot retrieve a representation.

It is tempting to guess the nature of a resource by inspection of a URI that identifies it. However, the Web is designed so that agents communicate resource information state through representations, not identifiers. In general, one cannot determine the type of a resource representation by inspecting a URI for that resource. For example, the ".html" at the end of "http://example.com/page.html" provides no guarantee that
representations of the identified resource will be served with the Internet media type "text/html". The publisher is free to allocate identifiers and define how they are served. The HTTP protocol does not constrain the Internet media type based on the path component of the URI; the URI owner is free to configure the server to return a representation using PNG or any other data format.

Resource state may evolve over time. Requiring a URI owner to publish a new URI for each change in resource state would lead to a significant number of broken references. For robustness, Web architecture promotes independence between an identifier and the state of the identified resource.

The example URI used in the travel scenario ("http://weather.example.com/oaxaca") suggests to a human reader that the identified resource has something to do with the weather in Oaxaca. A site reporting the weather in Oaxaca could just as easily be identified by the URI "http://vjc.example.com/315". And the URI "http://weather.example.com/vancouver" might identify the resource "my photo album."

On the other hand, the URI "mailto:joe@example.com" indicates that the URI refers to a mailbox. The "mailto" URI scheme specification authorizes agents to infer that URIs of this form identify Internet mailboxes.

Some URI assignment authorities document and publish their URI assignment policies. For more information about URI opacity, see TAG issues metaDataInURI-31 and siteData-36.

When browsing the XHTML document that Nadia receives as a representation of the resource identified by "http://weather.example.com/oaxaca", she finds that the URI "http://weather.example.com/oaxaca#weekend" refers to the part of the representation that conveys information about the weekend outlook. This URI includes the fragment identifier "weekend" (the string after the "#").

The fragment identifier component of a URI allows indirect identification of a secondary resource by reference to a primary resource and additional identifying information. The secondary resource may be some portion or subset of the primary resource, some view on representations of the primary resource, or some other resource defined or described by
those representations. The terms "primary resource" and "secondary resource" are defined in section 3.5 of [URI].

The terms “primary” and “secondary” in this context do not limit the nature of the resource—they are not classes. In this context, primary and secondary simply indicate that there is a relationship between the resources for the purposes of one URI: the URI with a fragment identifier. Any resource can be identified as a secondary resource. It might also be identified using a URI without a fragment identifier, and a resource may be identified as a secondary resource via
multiple URIs. The purpose of these terms is to enable discussion of the relationship between such resources, not to limit the nature of a resource.

The integration of internationalized identifiers (i.e., composed of characters beyond those allowed by [URI]) into the Web architecture is an important and open issue. See TAG issue IRIEverywhere-27 for discussion about work going on in this area.

Emerging Semantic Web technologies, including the "Web Ontology Language (OWL)" [OWL10], define RDF properties such as sameAs to assert that two URIs identify the same resource or inverseFunctionalProperty to imply it.

Communication between agents over a network about resources involves URIs, messages, and data. The Web's protocols (including HTTP, FTP, SOAP, NNTP, and SMTP) are based on the exchange of messages. A message may include data as well as metadata about a resource (such as the "Alternates" and "Vary" HTTP headers), the message data, and the message itself (such as the "Transfer-encoding" HTTP header). A message may even
include metadata about the message metadata (for message-integrity checks, for instance).

Story

Nadia follows a hypertext link labeled "satellite image" expecting to retrieve a satellite photo of the Oaxaca region. The link to the satellite image is an XHTML link encoded as <a href="http://example.com/satimage/oaxaca">satellite image</a>. Nadia's browser analyzes the URI and determines that its scheme is "http". The browser configuration determines how it locates the identified information, which might be
via a cache of prior retrieval actions, by contacting an intermediary (such as a proxy server), or by direct access to the server identified by a portion of the URI. In this example, the browser opens a network connection to port 80 on the server at "example.com" and sends a "GET" message as specified by the HTTP protocol, requesting a representation of the resource.

The server sends a response message to the browser, once again according to the HTTP protocol. The message consists of several headers and a JPEG image. The browser reads the headers, learns from the "Content-Type" field that the Internet media type of the representation is "image/jpeg", reads the sequence of octets that make up the representation data, and renders the image.

This section describes the architectural principles and constraints regarding interactions between agents, including such topics as network protocols and interaction styles, along with interactions between the Web as a system and the people that make use of it. The fact that the Web is a highly distributed system affects architectural constraints and assumptions about interactions.

Agents may use a URI to access the referenced resource; this is called dereferencing the URI. Access may take many forms, including retrieving a representation of the resource (for instance, by using HTTP GET or HEAD), adding or modifying a representation of the resource (for instance, by using HTTP POST or PUT, which in some cases may change the actual state of the resource if the submitted representations are
interpreted as instructions to that end), and deleting some or all representations of the resource (for instance, by using HTTP DELETE, which in some cases may result in the deletion of the resource itself).

There may be more than one way to access a resource for a given URI; application context determines which access method an agent uses. For instance, a browser might use HTTP GET to retrieve a representation of a resource, whereas a hypertext link checker might use HTTP HEAD on the same URI simply to establish whether a representation is available. Some URI schemes set expectations about available access methods, others (such as the URN scheme [RFC 2141]) do not. Section 1.2.2 of [URI] discusses the separation of identification and interaction in more detail. For more information about relationships between multiple access methods and URI addressability, see the TAG finding "URIs, Addressability, and the use of HTTP GET and POST".

Although many URI schemes (§2.4) are named after protocols, this does not imply that use of such a URI will necessarily result in access to the resource via the named protocol. Even when an agent uses a URI to retrieve a representation, that access might be through gateways, proxies, caches, and name resolution services that are independent of the protocol associated with the scheme name.

Many URI schemes define a default interaction protocol for attempting access to the identified resource. That interaction protocol is often the basis for allocating identifiers within that scheme, just as "http" URIs are defined in terms of TCP-based HTTP servers. However, this does not imply that all interaction with such resources is limited to the default interaction protocol. For example, information retrieval systems often make use of proxies to interact with a
multitude of URI schemes, such as HTTP proxies being used to access "ftp" and "wais" resources. Proxies can also to provide enhanced services, such as annotation proxies that combine normal information retrieval with additional metadata retrieval to provide a seamless, multidimensional view of resources using the same protocols and user agents as the non-annotated Web. Likewise, future protocols may be defined that encompass our current systems, using entirely different interaction mechanisms, without
changing the existing identifier schemes.

Dereferencing a URI generally involves a succession of steps as described in multiple specifications and implemented by the agent. The following example illustrates the series of specifications that govern the process when a user agent is instructed to follow a hypertext link (§4.4) that is part of an SVG document. In this example, the URI is "http://weather.example.com/oaxaca" and the application context calls for the user agent to
retrieve and render a representation of the identified resource.

Since the URI is part of a hypertext link in an SVG document, the first relevant specification is the SVG 1.1 Recommendation [SVG11]. Section 17.1 of this specification imports the link semantics defined in XLink 1.0 [XLink10]: "The remote resource (the destination for the link) is defined by a URI specified by the XLink href attribute on the 'a'
element." The SVG specification goes on to state that interpretation of an a element involves retrieving a representation of a resource, identified by the href attribute in the XLink namespace: "By activating these links (by clicking with the mouse, through keyboard input, voice commands, etc.), users may visit these resources."

The XLink 1.0 [XLink10] specification, which defines the href attribute in section 5.4, states that "The value of the href attribute must be a URI reference as defined in [IETF RFC 2396], or must result in a URI reference after the escaping procedure described below is applied."

The URI specification [URI] states that "Each URI begins with a scheme name that refers to a specification for assigning identifiers within that scheme." The URI scheme name in this example is "http".

[IANASchemes] states that the "http" scheme is defined by the HTTP/1.1 specification (RFC 2616 [RFC2616], section 3.2.2).

In this SVG context, the agent constructs an HTTP GET request (per section 9.3 of [RFC2616]) to retrieve the representation.

Section 6 of [RFC2616] defines how the server constructs a corresponding response message, including the 'Content-Type' field.

Section 1.4 of [RFC2616] states "HTTP communication usually takes place over TCP/IP connections." This example does not address that step in the process, or other steps such as Domain Name System (DNS) resolution.

The agent interprets the returned representation according to the data format specification that corresponds to the representation's Internet Media Type (§3.2) (the value of the HTTP 'Content-Type') in the relevant IANA registry [MEDIATYPEREG].

Precisely which representation(s) are retrieved depends on a number of factors, including:

If the URI owner has provided more than one representation (in different formats such as HTML, PNG, or RDF; in different languages such as English and Spanish; or transformed dynamically according to the hardware or software capabilities of the recipient), the resulting representation may depend on negotiation between the user agent and server.

The time of the request; the world changes over time, so representations of resources are also likely to change over time.

Assuming that a representation has been successfully retrieved, the expressive power of the representation’s format will affect how precisely the representation provider communicates resource state. If the representation communicates the state of the resource inaccurately, this inaccuracy or ambiguity may lead to confusion about what the resource is. If different users reach different conclusions about what the resource is, this may lead to URI collision (§2.2.1). Some communities, such as the ones developing the Semantic Web, seek to provide a framework for accurately communicating the semantics of a resource in a machine readable way. Machine readable semantics may alleviate some of the ambiguity associated with natural language descriptions of resources.

A Representation is data that encodes information about resource state. Representations do not necessarily describe the resource, or portray a likeness of the resource, or represent the resource in other senses of the word "represent".

Representations of a resource may be sent or received using interaction protocols. These protocols in turn determine the form in which representations are conveyed on the Web. HTTP, for example, provides for transmission of representations as octet streams typed using Internet media types [RFC2046].

Just as it is important to reuse existing schemes whenever possible, there are significant benefits to using media typed octet streams for representations even in the unusual case where a new scheme and associated protocol is to be defined. For example, if the Oaxaca weather were conveyed to Nadia's browser using a protocol other than HTTP, then software to render formats such as text/xhmtl+xml and image/png would still be usable if the new protocol supported transmission of
those types. This is an example of the principle of orthogonal specifications (§5.1).

The Internet Media Type defines the syntax and semantics of the fragment identifier, if any, that may be used in conjunction with a representation.

Story

In one of his XHTML pages, Dirk creates a hypertext link to an image that Nadia has published on the Web. He creates a hypertext link with <a href="http://www.example.com/images/nadia#hat">Nadia's hat</a>. Emma views Dirk's XHTML page in her Web browser and follows the link. The HTML implementation in her browser removes the fragment from the URI and requests the image "http://www.example.com/images/nadia". Nadia serves an SVG representation of the
image (with Internet media type "image/svg+xml"). Emma's Web browser starts up an SVG implementation to view the image. It passes it the original URI including the fragment, "http://www.example.com/images/nadia#hat" to this implementation, causing a view of the hat to be displayed rather than the complete image.

Note that the HTML implementation in Emma's browser did not need to understand the syntax or semantics of the SVG fragment (nor does the SVG implementation have to understand HTML, WebCGM, RDF ... fragment syntax or semantics; it merely had to recognize the # delimiter from the URI syntax [URI] and remove the fragment when requesting the resource). This orthogonality (§5.1) is an important feature of Web architecture; it
is what enabled Emma's browser to provide a useful service without requiring an upgrade.

The semantics of a fragment identifier are defined by the set of representations that might result from a retrieval action on the primary resource. The fragment's format and resolution are therefore dependent on the type of a potentially retrieved representation, even though such a retrieval is only performed if the URI is dereferenced. If no such representation exists, then the semantics of the fragment are considered unknown and, effectively, unconstrained. Fragment
identifier semantics are orthogonal to URI schemes and thus cannot be redefined by URI scheme specifications.

Interpretation of the fragment identifier is performed solely by the agent that dereferences a URI; the fragment identifier is not passed to other systems during the process of retrieval. This means that some intermediaries in Web architecture (such as proxies) have no interaction with fragment identifiers and that redirection (in HTTP [RFC2616], for example) does not account for fragments.

Content negotiation refers to the practice of making available multiple representations via the same URI. Negotiation between the requesting agent and the server determines which representation is served (usually with the goal of serving the "best" representation a receiving agent can process). HTTP is an example of a protocol that enables representation providers to use content negotiation.

Individual data formats may define their own rules for use of the fragment identifier syntax for specifying different types of subsets, views, or external references that are identifiable as secondary resources by that media type. Therefore, representation providers must manage content negotiation carefully when used with a URI that contains a fragment identifier. Consider an example where the owner of the URI "http://weather.example.com/oaxaca/map#zicatela" uses content
negotiation to serve two representations of the identified resource. Three situations can arise:

The interpretation of "zicatela" is defined consistently by both data format specifications. The representation provider decides when definitions of fragment identifier semantics are are sufficiently consistent.

The interpretation of "zicatela" is defined inconsistently by the data format specifications.

The interpretation of "zicatela" is defined in one data format specification but not the other.

The first situation—consistent semantics—poses no problem.

The second case is a server management error: representation providers must not use content negotiation to serve representation formats that have inconsistent fragment identifier semantics. This situation also leads to URI collision (§2.2.1).

The third case is not a server management error. It is a means by which the Web can grow. Because the Web is a distributed system in which formats and agents are deployed in a non-uniform manner, Web architecture does not constrain authors to only use "lowest common denominator" formats. Content authors may take advantage of new data formats while still ensuring reasonable backward-compatibility for agents that do not yet implement them.

In case three, behavior by the receiving agent should vary depending on whether the negotiated format defines fragment identifier semantics. When a received data format does not define fragment identifier semantics, the agent should not perform silent error recovery unless the user has given consent; see [CUAP] for additional suggested agent behavior in this case.

Successful communication between two parties depends on a reasonably shared understanding of the semantics of exchanged messages, both data and metadata. At times, there may be inconsistencies between a message sender's data and metadata. For instance, examples that have been observed in practice of inconsistencies between representation data and metadata include:

The actual character encoding of a representation (e.g., "iso-8859-1", specified by the encoding attribute in an XML declaration) is inconsistent with the charset parameter in the representation metadata (e.g., "utf-8", specified by the 'Content-Type' field in an HTTP header).

The namespace (§4.5.3) of the root element of XML representation data (e.g., as specified by the "xmlns" attribute) is inconsistent with the value of the 'Content-Type' field in an HTTP header.

On the other hand, there is no inconsistency in serving HTML content with the media type "text/plain", for example, as this combination is licensed by specifications.

Agents MUST NOT ignore message metadata without the consent of the user.

Thus, for example, if the parties responsible for "weather.example.com" mistakenly label the satellite photo of Oaxaca as "image/gif" instead of "image/jpeg", and if Nadia's browser detects a problem, Nadia's browser must not ignore the problem (e.g., by simply rendering the JPEG image) without Nadia's consent. Nadia's browser can notify Nadia of the problem or notify Nadia and take corrective action.

Furthermore, representation providers can help reduce the risk of inconsistencies through careful assignment of representation metadata (especially that which applies across representations). The section on media types for XML (§4.5.7) presents an example of reducing the risk of error by providing no metadata about character encoding when serving XML.

The accuracy of metadata relies on the server administrators, the authors of representations, and the software that they use. Practically, the capabilities of the tools and the social relationships may be the limiting factors.

The accuracy of these and other metadata fields is just as important for dynamic Web resources, where a little bit of thought and programming can often ensure correct metadata for a huge number of resources.

Often there is a separation of control between the users who create representations of resources and the server managers who maintain the Web site software. Given that it is generally the Web site software that provides the metadata associated with a resource, it follows that coordination between the server managers and content creators is required.

Nadia's retrieval of weather information (an example of a read-only query or lookup) qualifies as a "safe" interaction; a safe interaction is one where the agent does not incur any obligation beyond the interaction. An agent may incur an obligation through other means (such as by signing a contract). If an agent does not have an obligation before a safe interaction, it does not have that obligation
afterwards.

Other Web interactions resemble orders more than queries. These unsafe interactions may cause a change to the state of a resource and the user may be held responsible for the consequences of these interactions. Unsafe interactions include subscribing to a newsletter, posting to a list, or modifying a database. Note: In this context, the word "unsafe" does not necessarily mean
"dangerous"; the term "safe" is used in section 9.1.1 of [RFC2616] and "unsafe" is the natural opposite.

Story

Nadia decides to book a vacation to Oaxaca at "booking.example.com." She enters data into a series of online forms and is ultimately asked for credit card information to purchase the airline tickets. She provides this information in another form. When she presses the "Purchase" button, her browser opens another network connection to the server at "booking.example.com" and sends a message composed of form data using the POST method. This is an unsafe interaction; Nadia wishes to change the state of the system by exchanging money for airline tickets.

The server reads the POST request, and after performing the booking transaction returns a message to Nadia's browser that contains a representation of the results of Nadia's request. The representation data is in XHTML so that it can be saved or printed out for Nadia's records.

Note that neither the data transmitted with the POST nor the data received in the response necessarily correspond to any resource identified by a URI.

Safe interactions are important because these are interactions where users can browse with confidence and where agents (including search engines and browsers that pre-cache data for the user) can follow hypertext links safely. Users (or agents acting on their behalf) do not commit themselves to anything by querying a resource or following a hypertext link.

For instance, it is incorrect to publish a URI that, when followed as part of a hypertext link, subscribes a user to a mailing list. Remember that search engines may follow such hypertext links.

The fact that URI retrieval is safe does not imply that all safe interactions must be done through URI retrieval. At times, there may be good reasons (such as confidentiality requirements or practical limits on URI length) to conduct an otherwise safe operation using a mechanism generally reserved for unsafe operations (e.g., HTTP POST).

Nadia pays for her airline tickets online (through a POST interaction as described above). She receives a Web page with confirmation information and wishes to bookmark it so that she can refer to it when she calculates her expenses. Although Nadia can print out the results, or save them to a file, she would also like to bookmark them.

Transaction requests and results are valuable resources, and like all valuable resources, it is useful to be able to refer to them with a persistent URI (§3.5.1). However, in practice, Nadia cannot bookmark her commitment to pay (expressed via the POST request) or the airline company's acknowledgment and commitment to provide her with a flight (expressed via the response to the POST).

There are ways to improve the situation. For transaction requests, user agents can provide an interface for managing transactions where the user agent has incurred an obligation on behalf of the user. For transaction results, HTTP allows representation providers to associate a URI with the results of an HTTP POST request using the "Content-Location" header (described in section 14.14 of [RFC2616]).

Since Nadia finds the Oaxaca weather site useful, she emails a review to her friend Dirk recommending that he check out 'http://weather.example.com/oaxaca'. Dirk clicks on the resulting hypertext link in the email he receives and is frustrated by a 404 (not found). Dirk tries again the next day and receives a representation with "news" that is two-weeks old. He tries one more time the next day only to receive a representation that claims that the weather in Oaxaca is sunny,
even though his friends in Oaxaca tell him by phone that in fact it is raining. Dirk and Nadia conclude that the URI owners are unreliable or unpredictable. Although the URI owner has chosen the Web as a communication medium, the owner has lost two customers due to ineffective resource management.

A URI owner may supply zero or more authoritative representations of the resource identified by that URI. There is a benefit to the community in providing representations.

An application developer or specification author SHOULD NOT require networked retrieval to representations each time they are referenced.

Dereferencing a URI has a (potentially significant) cost in computing and bandwidth resources, may have security implications, and may impose significant latency on the dereferencing application. Dereferencing URIs should be avoided except when necessary.

As is the case with many human interactions, confidence in interactions via the Web depends on stability and predictability. For an information resource, persistence depends on the consistency of representations. The representation provider decides when representations are sufficiently consistent (although that determination generally takes user expectations into account).

Although persistence in this case is observable as a result of representation retrieval, the term URI persistence is used to describe the desirable property that, once associated with a resource, a URI should continue indefinitely to refer to that resource.

A URI owner SHOULD provide representations of the identified resource consistently and predictably.

URI persistence is a matter of policy and commitment on the part of the URI owner. The choice of a particular URI scheme provides no guarantee that those URIs will be persistent or that they will not be persistent.

HTTP [RFC2616] has been designed to help manage URI persistence. For example, HTTP redirection (using the 3xx response codes) permits servers to tell an agent that further action needs to be taken by the agent in order to fulfill the request (for example, a new URI is associated with the resource).

In addition, content negotiation also promotes consistency, as a site manager is not required to define new URIs when adding support for a new format specification. Protocols that do not support content negotiation (such as FTP) require a new identifier when a new data format is introduced. Improper use of content negotiation can lead to inconsistent representations.

It is reasonable to limit access to a resource (for commercial or security reasons, for example), but merely identifying the resource is like referring to a book by title. In exceptional circumstances, people may have agreed to keep titles or URIs confidential (for example, a book author and a publisher may agree to keep the URI of page containing additional material secret until after the book is published), otherwise they are free to exchange them.

As an analogy: The owners of a building might have a policy that the public may only enter the building via the main front door, and only during business hours. People who work in the building and who make deliveries to it might use other doors as appropriate. Such a policy would be enforced by a combination of security personnel and mechanical devices such as locks and pass-cards. One would not enforce this policy by hiding some of the building entrances, nor by requesting
legislation requiring the use of the front door and forbidding anyone to reveal the fact that there are other doors to the building.

Story

Nadia sends to Dirk the URI of the current article she is reading. With his browser, Dirk follows the hypertext link and is asked to enter his subscriber username and password. Since Dirk is also a subscriber to services provided by "weather.example.com," he can access the same information as Nadia. Thus, the authority for "weather.example.com" can limit access to authorized parties and still provide the benefits of URIs.

The Web provides several mechanisms to control access to resources; these mechanisms do not rely on hiding or suppressing URIs for those resources. For more information, see the TAG finding "'Deep Linking' in the World Wide Web".

It is a strength of Web Architecture that links can be made and shared; a user who has found an interesting part of the Web can share this experience just by republishing a URI.

Story

Nadia and Dirk want to visit the Museum of Weather Forecasting in Oaxaca. Nadia goes to "http://maps.example.com", locates the museum, and mails the URI "http://maps.example.com/oaxaca?lat=17.065;lon=-96.716;scale=6" to Dirk. Dirk goes to "http://mymaps.example.com", locates the museum, and mails the URI "http://mymaps.example.com/geo?sessionID=765345;userID=Dirk" to Nadia. Dirk reads Nadia's email and is able to follow the link to the map. Nadia reads Dirk's email, follows
the link, and receives an error message 'No such session/user'. Nadia has to start again from "http://mymaps.example.com" and find the museum location once more.

For resources that are generated on demand, machine generation of URIs is common. For resources that might usefully be bookmarked for later perusal, or shared with others, server managers should avoid needlessly restricting the reusability of such URIs. If the intention is to restrict information to a particular user, as might be the case in a home banking application for example, designers should use appropriate access control (§3.5.2)
mechanisms.

Interactions conducted with HTTP POST (where HTTP GET could have been used) also limit navigation possibilities. The user cannot create a bookmark or share the URI with others since the URI seen by the user does not change as the user moves from page to page.

There remain open questions regarding Web interactions. The TAG expects future versions of this document to address in more detail the relationship between the architecture described herein, Web Services, peer-to-peer systems, instant messaging systems (such as [RFC3920]), streaming audio (such as RTSP [RFC2326]), and voice-over-IP (such as SIP
[RFC3261]).

A data format specification (for example, for XHTML, RDF/XML, SMIL, XLink, CSS, and PNG) embodies an agreement on the correct interpretation of representation data. The first data format used on the Web was HTML. Since then, data formats have grown in number. Web architecture does not constrain which data formats content providers can use. This flexibility is important because there is constant evolution in applications,
resulting in new data formats and refinements of existing formats. Although Web architecture allows for the deployment of new data formats, the creation and deployment of new formats (and agents able to handle them) is expensive. Thus, before inventing a new data format (or "meta" format such as XML), designers should carefully consider re-using one that is already available.

For a data format to be usefully interoperable between two parties, the parties must agree (to a reasonable extent) about its syntax and semantics. Shared understanding of a data format promotes interoperability but does not imply constraints on usage; for instance, a sender of data cannot count on being able to constrain the behavior of a data receiver.

Below we describe some characteristics of a data format that facilitate integration into Web architecture. This document does not address generally beneficial characteristics of a specification such as readability, simplicity, attention to programmer goals, attention to user needs, accessibility, nor internationalization. The section on architectural specifications (§7.1) includes references to additional format specification
guidelines.

Binary data formats are those in which portions of the data are encoded for direct use by computer processors, for example 32 bit little-endian two's-complement and 64 bit IEEE double-precision floating-point. The portions of data so represented include numeric values, pointers, and compressed data of all sorts.

A textual data format is one in which the data is specified in a defined encoding as a sequence of characters. HTML, Internet e-mail, and all XML-based formats (§4.5) are textual. Increasingly, internationalized textual data formats refer to the Unicode repertoire [UNICODE] for character definitions.

If a data format is textual, as defined in this section, that does not imply that it should be served with a media type beginning with "text/". Although XML-based formats are textual, many XML-based formats do not consist primarily of phrases in natural language. See the section on media types for XML (§4.5.7) for issues that arise when "text/" is used in conjunction with an XML-based format.

In principle, all data can be represented using textual formats. In practice, some types of content (e.g., audio and video) are generally represented using binary formats.

The trade-offs between binary and textual data formats are complex and application-dependent. Binary formats can be substantially more compact, particularly for complex pointer-rich data structures. Also, they can be consumed more rapidly by agents in those cases where they can be loaded into memory and used with little or no conversion. Note, however, that such cases are relatively uncommon as such direct use may open the door to security issues that can only practically be
addressed by examining every aspect of the data structure in detail.

Textual formats are usually more portable and interoperable. Textual formats also have the considerable advantage that they can be directly read by human beings (and understood, given sufficient documentation). This can simplify the tasks of creating and maintaining software, and allow the direct intervention of humans in the processing chain without recourse to tools more complex than the ubiquitous text editor. Finally, it simplifies the necessary human task of learning
about new data formats; this is called the "view source" effect.

It is important to emphasize that intuition as to such matters as data size and processing speed is not a reliable guide in data format design; quantitative studies are essential to a correct understanding of the trade-offs. Therefore, designers of a data format specification should make a considered choice between binary and textual format design.

In a perfect world, language designers would invent languages that perfectly met the requirements presented to them, the requirements would be a perfect model of the world, they would never change over time, and all implementations would be perfectly interoperable because the specifications would have no variability.

In the real world, language designers imperfectly address the requirements as they interpret them, the requirements inaccurately model the world, conflicting requirements are presented, and they change over time. As a result, designers negotiate with users, make compromises, and often introduce extensibility mechanisms so that it is possible to work around problems in the short term. In the long term, they produce multiple versions of their languages, as the problem, and
their understanding of the problem, evolves. The resulting variability in specifications, languages, and implementations introduces interoperability costs.

Extensibility and versioning are strategies to help manage the natural evolution of information on the Web and technologies used to represent that information. For more information about how these strategies introduce variability and how that variability impacts interoperability, see Variability in Specifications.

See TAG issue XMLVersioning-41, which concerns good practices for designing extensible XML languages and for handling versioning. See also "Web Architecture: Extensible Languages" [EXTLANG].

Nadia and Dirk are designing an XML data format to encode data about the film industry. They provide for extensibility by using XML namespaces and creating a schema that allows the inclusion, in certain places, of elements from any namespace. When they revise their format, Nadia proposes a new optional lang attribute on the film element. Dirk feels that such a change requires them to assign a new namespace name, which might require changes to
deployed software. Nadia explains to Dirk that their choice of extensibility strategy in conjunction with their namespace policy allows certain changes that do not affect conformance of existing content and software, and thus no change to the namespace identifier is required. They chose this policy to help them meet their goals of reducing the cost of change.

Dirk and Nadia have chosen a particular namespace change policy that allows them to avoid changing the namespace name whenever they make changes that do not affect conformance of deployed content and software. They might have chosen a different policy, for example that any new element or attribute has to belong to a namespace other than the original one. Whatever the chosen policy, it should set clear expectations for users of the format.

In general, changing the namespace name of an element completely changes the element name. If "a" and "b" are bound to two different URIs, a:element and b:element are as distinct as a:eieio and a:xyzzy. Practically speaking, this means that deployed applications will have to be upgraded in order to recognize the new language; the cost of this upgrade may be very high.

It follows that there are significant tradeoffs to be considered when deciding on a namespace change policy. If a vocabulary has no extensibility points (that is, if it does not allow elements or attributes from foreign namespaces or have a mechanism for dealing with unrecognized names from the same namespace), it may be absolutely necessary to change the namespace name. Languages that allow some form of extensibility without requiring a change to the namespace name are more
likely to evolve gracefully.

An XML format specification SHOULD include information about change policies for XML namespaces.

As an example of a change policy designed to reflect the variable stability of a namespace, consider the W3C namespace policy for documents on the W3C Recommendation track. The policy sets expectations that the Working Group responsible for the namespace may modify it in any way until a certain point in the process ("Candidate Recommendation") at which point W3C constrains the set of possible changes to the namespace
in order to promote stable implementations.

Note that since namespace names are URIs, the owner of a namespace URI has the authority to decide the namespace change policy.

Requirements change over time. Successful technologies are adopted and adapted by new users. Designers can facilitate the transition process by making careful choices about extensibility during the design of a language or protocol specification.

In making these choices, the designers must weigh the trade-offs between extensibility, simplicity, and variability. A language without extensibility mechanisms may be simpler and less variable, improving initial interoperability. However, it’s likely that changes to that language will be more difficult, possibly more complex and more variable, than if the initial design had provided such mechanisms. This may decrease interoperability over the long term.

A specification SHOULD provide mechanisms that allow any party to create extensions.

Extensibility introduces variability which has an impact on interoperability. However, languages that have no extensibility mechanisms may be extended in ad hoc ways that impact interoperability as well. One key criterion of the mechanisms provided by language designers is that they allow the extended languages to remain conformant to the original specification, increasing the likelihood of interoperability.

Extensibility MUST NOT interfere with conformance to the original specification.

Application needs determine the most appropriate extension strategy for a specification. For example, applications designed to operate in closed environments may allow specification designers to define a versioning strategy that would be impractical at the scale of the Web.

A powerful design approach is for the language to allow either form of extension, but to distinguish explicitly between them in the syntax.

Additional strategies include prompting the user for more input and automatically retrieving data from available hypertext links. More complex strategies are also possible, including mixing strategies. For instance, a language can include mechanisms for overriding standard behavior. Thus, a data format can specify "must ignore" semantics but also allow for extensions that override that semantics in light of application needs (for instance, with "must understand" semantics
for a particular extension).

Extensibility is not free. Providing hooks for extensibility is one of many requirements to be factored into the costs of language design. Experience suggests that the long term benefits of a well-designed extensibility mechanism generally outweigh the costs.

In principle, these relationships can be mixed and nested arbitrarily. A SOAP message, for example, can contain an SVG image that contains an RDF comment which refers to a vocabulary of terms for describing the image.

Note however, that for general XML there is no semantic model that defines the interactions within XML documents with elements and/or attributes from a variety of namespaces. Each application must define how namespaces interact and what effect the namespace of an element has on the element's ancestors, siblings, and descendants.

See TAG issues mixedUIXMLNamespace-33 (concerning the meaning of a document composed of content in multiple namespaces), xmlFunctions-34 (concerning one approach for managing XML transformation and composability), and RDFinXHTML-35 (concerning the interpretation of RDF when embedded in an XHTML document).

The Web is a heterogeneous environment where a wide variety of agents provide access to content to users with a wide variety of capabilities. It is good practice for authors to create content that can reach the widest possible audience, including users with graphical desktop computers, hand-held devices and mobile phones, users with disabilities who may require speech synthesizers, and devices not yet imagined. Furthermore, authors cannot predict in some cases how an agent
will display or process their content. Experience shows that the separation of content, presentation, and interaction promotes the reuse and device-independence of content; this follows from the principle of orthogonal specifications (§5.1).

This separation also facilitates reuse of authored source content across multiple delivery contexts. Sometimes, functional user experiences suited to any delivery context can be generated by using an adaptation process applied to a representation that does not depend on the access mechanism. For more information about principles of device-independence, see [DIPRINCIPLES].

A specification SHOULD allow authors to separate content from both presentation and interaction concerns.

Note that when content, presentation, and interaction are separated by design, agents need to recombine them. There is a recombination spectrum, with "client does all" at one end and "server does all" at the other. There are advantages to each: recombination on the server allows the server to send out generally smaller amounts of data that can be tailored to specific devices (such as mobile phones). However, such data will not be readily reusable by other clients and may not
allow client-side agents to perform useful tasks unanticipated by the author. When a client does the work of recombination, content is likely to be more reusable by a broader audience and more robust. However, such data may be of greater size and may require more computation by the client.

The design decision about where on this spectrum an application should be placed depends on the power on the client, the power and the load on the server, and the bandwidth of the medium that connects them. If the number of possible clients is unbounded, the application will scale better if more computation is pushed to the client.

Of course, it may be desirable to limit the audience. Designers should consider appropriate technologies, such as encryption and access control (§3.5.2), for limiting the audience.

Some data formats are designed to describe presentation (including SVG and XSL Formatting Objects). Data formats such as these demonstrate that one can only separate content from presentation (or interaction) so far; at some point it becomes necessary to talk about presentation. Per the principle of orthogonal specifications (§5.1) these data formats should only address presentation issues.

A defining characteristic of the Web is that it allows embedded references to other resources via URIs. The simplicity of creating hypertext links using absolute URIs (<a href="http://www.example.com/foo">) and relative URI references (<a href="foo"> and <a href="foo#anchor">) is partly (perhaps largely) responsible for the success of the hypertext Web as we know it today.

When one resource (representation) refers to another resource with a URI, this constitutes a link between the two resources. Additional metadata may also form part of the link (see [XLink10], for example). Note: In this document, the term "link" generally means "relationship", not "physical connection".

Links are commonly expressed using URI references (defined in section 4.2 of [URI]), which may be combined with a base URI to yield a usable URI. Section 5.1 of [URI] explains different ways to establish a base URI for a resource and establishes a precedence among them. For instance, the base URI may be a URI for the resource, or specified in a representation (see the
base elements provided by HTML and XML, and the HTTP 'Content-Location' header). See also the section on links in XML (§4.5.2).

Agents resolve a URI reference before using the resulting URI to interact with another agent. URI references help in content management by allowing content authors to design a representation locally, i.e., without concern for which global identifier may later be used to refer to the associated resource.

Many data formats are XML-based, that is to say they conform to the syntax rules defined in the XML specification [XML10] or [XML11]. This section discusses issues that are specific to such formats. Anyone seeking guidance in this area is urged to consult the "Guidelines For the Use of XML in IETF Protocols" [IETFXML],
which contains a thorough discussion of the considerations that govern whether or not XML ought to be used, as well as specific guidelines on how it ought to be used. While it is directed at Internet applications with specific reference to protocols, the discussion is generally applicable to Web scenarios as well.

The discussion here should be seen as ancillary to the content of [IETFXML]. Refer also to "XML Accessibility Guidelines" [XAG] for help designing XML formats that lower barriers to Web accessibility for people with disabilities.

XML defines textual data formats that are naturally suited to describing data objects which are hierarchical and processed in a chosen sequence. It is widely, but not universally, applicable for data formats; an audio or video format, for example, is unlikely to be well suited to expression in XML. Design constraints that would suggest the use of XML include:

Requirement for a hierarchical structure.

Immediate need for a wide range of tools on a variety of platforms.

Need for data that can outlive the applications that currently process it.

Ability to support internationalization in a self-describing way that makes confusion over coding options unlikely.

Early detection of encoding errors with no requirement to "work around" such errors.

A high proportion of human-readable textual content.

Potential composition of the data format with other XML-encoded formats.

Desire for data easily parsed by both humans and machines.

Desire for vocabularies that can be invented in a distributed manner and combined flexibly.

Sophisticated linking mechanisms have been invented for XML formats. XPointer allows links to address content that does not have an explicit, named anchor. [XLink] is an appropriate specification for representing links in hypertext (§4.4) XML applications. XLink allows links to have multiple ends and to be expressed either inline or in "link bases" stored external to any or all of the resources
identified by the links it contains.

Designers of XML-based formats should consider using XLink and, for defining fragment identifier syntax, using the XPointer framework and XPointer element() Schemes.

XLink is not the only linking design that has been proposed for XML, nor is it universally accepted as a good design. See also TAG issue xlinkScope-23.

The purpose of an XML namespace (defined in [XMLNS]) is to allow the deployment of XML vocabularies (in which element and attribute names are defined) in a global environment and to reduce the risk of name collisions in a given document when vocabularies are combined. For example, the MathML and SVG specifications both define the set element. Although XML data from different formats such as MathML and SVG can be combined in a
single document, in this case there could be ambiguity about which set element was intended. XML namespaces reduce the risk of name collisions by taking advantage of existing systems for allocating globally scoped names: the URI system (see also the section on URI allocation (§2.2.2)). When using XML namespaces, each local name in an XML vocabulary is paired with a URI (called the namespace URI) to distinguish the local name from
local names in other vocabularies.

The use of URIs confers additional benefits. First, each URI/local name pair can be mapped to another URI, grounding the terms of the vocabulary in the Web. These terms may be important resources and thus it is appropriate to be able to associate URIs with them.

For flat namespaces, concatenation is one useful mapping. If namespace URIs that end with a hash (“#”) are chosen, then simple concatenation of the namespace URI and the local name creates a URI for a secondary resource (the identified term). This technique is used for many [RDFXML] namespaces.

Other mappings are likely to be more suitable for hierarchical namespaces; see the related TAG issue abstractComponentRefs-37.

Designers of XML-based data formats who declare namespaces thus make it possible to reuse those data formats and combine them in novel ways not yet imagined. Failure to declare namespaces makes such re-use more difficult, even impractical in some cases.

A specification that establishes an XML vocabulary SHOULD place all element names and global attribute names in a namespace.

Attributes are always scoped by the element on which they appear. An attribute that is "global," that is, one that might meaningfully appear on elements of many types, including elements in other namespaces, should be explicitly placed in a namespace. Local attributes, ones associated with only a particular element type, need not be included in a namespace since their meaning will always be clear from the context provided by that element.

The type attribute from the W3C XML Schema Instance namespace "http://www.w3.org/2001/XMLSchema-instance" ([XMLSCHEMA], section 4.3.2) is an example of a global attribute. It can be used by authors of any vocabulary to make an assertion in instance data about the type of the element on which it appears. As a global attribute, it must always be qualified. The frame attribute on an HTML table is an example of a
local attribute. There is no value in placing that attribute in a namespace since the attribute is unlikely to be useful on an element other than an HTML table.

Applications that rely on DTD processing must impose additional constraints on the use of namespaces. DTDs perform validation based on the lexical form of the element and attribute names in the document. This makes prefixes syntactically significant in ways that are not anticipated by [XMLNS].

Nadia receives representation data from "weather.example.com" in an unfamiliar data format. She knows enough about XML to recognize which XML namespace the elements belong to. Since the namespace is identified by the URI "http://weather.example.com/2003/format", she asks her browser to retrieve a representation of the identified resource. She gets back some useful data that allows her to learn more about the data format. Nadia's browser may also be able to perform some
operations automatically (i.e., unattended by a human overseer) given data that has been optimized for software agents. For example, her browser might, on Nadia's behalf, download additional agents to process and render the format.

Another benefit of using URIs to build XML namespaces is that the namespace URI can be used to identify an information resource that contains useful information, machine-usable and/or human-usable, about terms in the namespace. This type of information resource is called a namespace document. When a namespace URI owner provides a namespace document, it is authoritative for the namespace.

There are many reasons to provide a namespace document. A person might want to:

understand the purpose of the namespace,

learn how to use the markup vocabulary in the namespace,

find out who controls it and associated policies,

request authority to access schemas or collateral material about it, or

report a bug or situation that could be considered an error in some collateral material.

A processor might want to:

retrieve a schema, for validation,

retrieve a style sheet, for presentation, or

retrieve ontologies, for making inferences.

In general, there is no established best practice for creating representations of a namespace document; application expectations will influence what data format or formats are used. Application expectations will also influence whether relevant information appears directly in a representation or is referenced from it.

The owner of an XML namespace name SHOULD make available material intended for people to read and material optimized for software agents in order to meet the needs of those who will use the namespace vocabulary.

For example, the following are examples of data formats for namespace documents: [OWL10], [RDDL], [XMLSCHEMA], and [XHTML11]. Each of these formats meets different requirements described above for satisfying the needs of an agent that wants more information about the namespace. Note, however, issues related to fragment identifiers and content negotiation (§3.2.2) if content negotiation is used.

See TAG issues namespaceDocument-8 (concerning desired characteristics of namespace documents) and abstractComponentRefs-37 (concerning the use of fragment identifiers with namespace names to identify abstract components).

Section 3 of "Namespaces in XML" [XMLNS] provides a syntactic construct known as a QName for the compact expression of qualified names in XML documents. A qualified name is a pair consisting of a URI, which names a namespace, and a local name placed within that namespace. "Namespaces in XML" provides for the use of QNames as names for XML elements and attributes.

Other specifications, starting with [XSLT10], have employed the idea of using QNames in contexts other than element and attribute names, for example in attribute values and in element content. However, general XML processors cannot reliably recognize QNames as such when they are used in attribute values and in element content; for example, the syntax of QNames overlaps with that of URIs. Experience has also revealed other limitations to
QNames, such as losing namespace bindings after XML canonicalization.

Because QNames are compact, some specification designers have adopted the same syntax as a means of identifying resources. Though convenient as a shorthand notation, this usage has a cost. There is no single, accepted way to convert a QName into a URI or vice versa. Although QNames are convenient, they do not replace the URI as the identification system of the Web. The use of QNames to identify Web resources without providing a mapping to URIs is inconsistent with Web
architecture.

Consider the following fragment of XML: <section name="foo">. Does the section element have what the XML Recommendation refers to as the ID foo (i.e., "foo" must not appear in the surrounding XML document more than once)? One cannot answer this question by examining the element and its attributes alone. In XML, the quality of "being an ID" is associated with the type of an attribute, not its name. Finding the IDs in a document
requires additional processing.

Processing the document with a processor that recognizes DTD attribute list declarations (in the external or internal subset) might reveal a declaration that identifies the name attribute as an ID. Note: This processing is not necessarily part of validation. A non-validating, DTD-aware processor can recognize IDs.

Processing the document with a W3C XML schema might reveal an element declaration that identifies the name attribute as an W3C XML Schema ID.

In practice, processing the document with another schema language, such as RELAX NG [RELAXNG], might reveal the attributes declared to be of ID in the XML Schema sense. Many modern specifications begin processing XML at the Infoset [INFOSET] level and do not specify normatively how an Infoset is constructed. For those specifications, any process that establishes the ID type in the Infoset (and Post Schema Validation Infoset
(PSVI) defined in [XMLSCHEMA]) may usefully identify the attributes of type ID.

In practice, applications may have independent means (such as those defined in the XPointer specification, [XPTRFR] section 3.2) of locating identifiers inside a document.

To further complicate matters, DTDs establish the ID type in the Infoset whereas W3C XML Schema produces a PSVI but does not modify the original Infoset. This leaves open the possibility that a processor might only look in the Infoset and consequently would fail to recognize schema-assigned IDs.

See the TAG issue xmlIDSemantics-32 for additional background information and [xml:id] for a solution under development.

There are two problems associated with the “text” media types: First, for data identified as "text/*", Web intermediaries are allowed to "transcode", i.e., convert one character encoding to another. Transcoding may make the self-description false or may cause the document to be not well-formed.

In general, a representation provider SHOULD NOT assign Internet media types beginning with "text/" to XML representations.

Second, representations whose Internet media types begin with "text/" are required, unless the charset parameter is specified, to be considered to be encoded in US-ASCII. Since the syntax of XML is designed to make documents self-describing, it is good practice to omit the charset parameter, and since XML is very often not encoded in US-ASCII, the use of "text/" Internet media types effectively precludes this good practice.

The section on media types and fragment identifier semantics (§3.2.1) discusses the interpretation of fragment identifiers. Designers of an XML-based data format specification should define the semantics of fragment identifiers in that format. The XPointer Framework [XPTRFR] provides an interoperable starting point.

When the media type assigned to representation data is "application/xml", there are no semantics defined for fragment identifiers, and authors should not make use of fragment identifiers in such data. The same is true if the assigned media type has the suffix "+xml" (defined in "XML Media Types" [RFC3023]), and the data format specification does not specify fragment identifier semantics. In short, just knowing that content is XML does not
provide information about fragment identifier semantics.

Many people assume that the fragment identifier #abc, when referring to XML data, identifies the element in the document with the ID "abc". However, there is no normative support for this assumption. A revision of RFC 3023 is expected to address this.

4.6. Future Directions for Data Formats

Data formats enable the creation of new applications to make use of the information space infrastructure. The Semantic Web is one such application, built on top of RDF [RDFXML]. This document does not discuss the Semantic Web in detail; the TAG expects that future editions of this document will. See the related TAG issue httpRange-14.

Identification, interaction, and representation are orthogonal concepts, meaning that technologies used for identification, interaction, and representation may evolve independently. For instance:

Resources are identified with URIs. URIs can be published without building any representations of the resource or determining whether any representations are available.

A generic URI syntax allows agents to function in many cases without knowing specifics of URI schemes.

In many cases one may change the representation of a resource without disrupting references to the resource (for example, by using content negotiation).

When two specifications are orthogonal, one may change one without requiring changes to the other, even if one has dependencies on the other. For example, although the HTTP specification depends on the URI specification, the two may evolve independently. This orthogonality increases the flexibility and robustness of the Web. For example, one may refer by URI to an image without knowing anything about the format chosen to represent the image. This has facilitated the
introduction of image formats such as PNG and SVG without disrupting existing references to image resources.

Experience demonstrates that problems arise where orthogonal concepts occur in a single specification. Consider, for example, the HTML specification which includes the orthogonal x-www-form-urlencoded specification. Software developers (for example, of [CGI] applications) might have an easier time finding the specification if it were published separately and then cited from the HTTP, URI, and HTML specifications.

Problems also arise when specifications attempt to modify orthogonal abstractions described elsewhere. An historical version of the HTML specification added a "Refresh" value to the http-equiv attribute of the meta element. It was defined to be equivalent to the HTTP header of the same name. The authors of the HTTP specification ultimately
decided not to provide this header and that made the two specifications awkwardly at odds with each other. The W3C HTML Working Group eventually removed the "Refresh" value.

A specification should clearly indicate which features overlap with those governed by another specification.

The information in the Web and the technologies used to represent that information change over time. Extensibility is the property of a technology that promotes evolution without sacrificing interoperability. Some examples of successful technologies designed to allow change while minimizing disruption include:

the fact that URI schemes are orthogonally specified;

the use of an open set of Internet media types in mail and HTTP to specify document interpretation;

the separation of the generic XML grammar and the open set of XML namespaces for element and attribute names;

An example of an unsuccessful extension mechanism is HTTP mandatory extensions [HTTPEXT]. The community has sought mechanisms to extend HTTP, but apparently the costs of the mandatory extension proposal (notably in complexity) outweighed the benefits and thus hampered adoption.

Below we discuss the property of "extensibility," exhibited by URIs, some data formats, and some protocols (through the incorporation of new messages).

Subset language: one language is a subset (or "profile") of a second language if any document in the first language is also a valid document in the second language and has the same interpretation in the second language.

Extended language: If one language is a subset of another, the latter superset is called an extended language; the difference between the languages is called the extension. Clearly, extending a language is better for interoperability than creating an incompatible language.

Ideally, many instances of a superset language can be safely and usefully processed as though they were in the subset language. Languages that can evolve this way, allowing applications to provide new information when necessary while still interoperating with applications that only understand a subset of the current language, are said to be "extensible." Language designers can facilitate extensibility by defining the default behavior of unknown extensions—for example, that
they be ignored (in some defined way) or should be considered errors.

For example, from early on in the Web, HTML agents followed the convention of ignoring unknown tags. This choice left room for innovation (i.e., non-standard elements) and encouraged the deployment of HTML. However, interoperability problems arose as well. In this type of environment, there is an inevitable tension between interoperability in the short term and the desire for extensibility. Experience shows that designs that strike the right balance between allowing change
and preserving interoperability are more likely to thrive and are less likely to disrupt the Web community. Orthogonal specifications (§5.1) help reduce the risk of disruption.

Errors occur in networked information systems. An error condition can be well-characterized (e.g., well-formedness errors in XML or 4xx client errors in HTTP) or arise unpredictably. Error correction means that an agent repairs a condition so that within the system, it is as though the error never occurred. One example of error correction involves data retransmission in response to a temporary network
failure. Error recovery means that an agent does not repair an error condition but continues processing by addressing the fact that the error has occurred.

Agents frequently correct errors without user awareness, sparing users the details of complex network communications. On the other hand, it is important that agents recover from error in a way that is evident to users, since the agents are acting on their behalf.

Agents that recover from error by making a choice without the user's consent are not acting on the user's behalf.

An agent is not required to interrupt the user (e.g., by popping up a confirmation box) to obtain consent. The user may indicate consent through pre-selected configuration options, modes, or selectable user interface toggles, with appropriate reporting to the user when the agent detects an error. Agent developers should not ignore usability issues when designing error recovery behavior.

To promote interoperability, specification designers should identify predictable error conditions. Experience has led to the following observations about error-handling approaches.

Protocol designers should provide enough information about an error condition so that an agent can address the error condition. For instance, an HTTP 404 status code (not found) is useful because it allows user agents to present relevant information to users, enabling them to contact the representation provider in case of problems.

Experience with the cost of building a user agent to handle the diverse forms of ill-formed HTML content convinced the designers of the XML specification to require that agents fail upon encountering ill-formed content. Because users are unlikely to tolerate such failures, this design choice has pressured all parties into respecting XML's constraints, to the benefit of all.

An agent that encounters unrecognized content may handle it in a number of ways, including by considering it an error; see also the section on extensibility and versioning (§4.2).

Error behavior that is appropriate for a person may not be appropriate for software. People are capable of exercising judgement in ways that software applications generally cannot. An informal error response may suffice for a person but not for a processor.

The Web follows Internet tradition in that its important interfaces are defined in terms of protocols, by specifying the syntax, semantics, and sequencing constraints of the messages interchanged. Protocols designed to be resilient in the face of widely varying environments have helped the Web scale and have facilitated communication across multiple trust boundaries. Traditional application programming interfaces (APIs) do not always take these constraints
into account, nor should they be required to. One effect of protocol-based design is that the technology shared among agents often lasts longer than the agents themselves.

It is common for programmers working with the Web to write code that generates and parses these messages directly. It is less common, but not unusual, for end users to have direct exposure to these messages. It is often desirable to provide users with access to format and protocol details: allowing them to “view source,” whereby they may gain expertise in the workings of the underlying system.

The practice of providing multiple representations available via the same URI. Which representation is served depends on negotiation between the requesting agent and the agent serving the representations.

Cool URIs don't change T. Berners-Lee, W3C, 1998 Available at http://www.w3.org/Provider/Style/URI. Note that the title is somewhat misleading. It is not the URIs that change, it is what they identify.

Representational State Transfer (REST), Chapter 5 of "Architectural Styles and the Design of Network-based Software Architectures", Doctoral Thesis of R. T. Fielding, 2000. Designers of protocol specifications in particular should invest time in understanding the REST model and the relevance of its principles to a given design. These principles include statelessness, clear assignment of roles
to parties, uniform address space, and a limited, uniform set of verbs. Available at http://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm.

Principled Design of the Modern Web Architecture, R.T. Fielding and R.N. Taylor, UC Irvine. In Proceedings of the 2000 International Conference on Software Engineering (ICSE 2000), Limerick, Ireland, June 2000, pp. 407-416. This document is available at http://www.ics.uci.edu/~fielding/pubs/webarch_icse2000.pdf.